Comparison of stubs_entry.S file, 1.00c and 1.00a firmware shows that both firmwares are aligned, so porting to one of them should work in other.
FWIW, on all the recent ports I've worked on, I've taken a few extra minutes to make an accurate CHDK-PT address file of the code segments used in the task files. That way I can usually port to different firmware versions by comparing the output of CHDK-PT for each version and then only tweaking the lines that change in the boot.c, capt_seq.c & movie_rec.c files. Typically the other files in the firmware directory don't change although its fairly quick to check those IF you documented where you found each value during the initial port.
Using that technique, you can really quickly check a different firmware version for compatability. Just run the CHDK-PT address file on your existing PRIMARY.BIN file, then the same address file on the new PRIMARY.BIN file. Use diff on the resultant code files to see if any of the task files change.
Some days ago I tried to port to 100c, but I had problems with booting.
There is typically a single subroutine call to something in high memory in the first code segment in boot.c that changes from firmware version to firmware version. If its above the top address that the sigfinder looks for the stubs_entry.S files will match but it still won't boot.